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The 5770 wont be as fast as the 4890, you'll be paying for the featureset..
2x 5770s are a popular option because they're comparable to a 5850 in performance, but cheaper overall..
Grab a GTX260 and fold on it aswell?
If buying new get a 5770 if buying second hand get a 4890 but don't pay more than £90 for one or even better deal.
Way I still look at it is i'd rather play dx11 game in dx9-10 with 60 fps than play it in dx11 with 30 fps.